The centriole is poorly understood and its role in the centrosome remains controversial. The pinwheel of nine triplet microtubules (MT's) is the prominent structural feature of the centriole that has, to a large extent, defined the concept of this organelle. Using correlative light and serial thick section HVEM/IVEM of sea urchin zygotes, we want to directly determine what role these triplet microtubules have in centrosome formation, duplication and the assembly and patterning of daughter cells.